LiveConnect allows ECMAscript code running Mozilla's Rhino interpreter to access Java code in a transparent way.
This tag should be used in questions specific to Mozilla Rhino ECMAscript interpreter that relates to ECMAscript or JavaScript AND Java. It's a way to embed Java code in your JavaScript scripts.
You can check Embedding Rhino or Scripting Java for tutorials.
Note that the Rhino interpreter can sandbox the interpreted code to disallow this functionality. You won't see this active in a browser since it poses security risks. That said, it's a very powerful technology that enables a lot of possibility for projects using Rhino.